1. Field of the Invention
This invention relates generally to a method for detecting leaks in a gas feeding system and, more particularly, to a method for detecting leaks in a supply line between a compressed hydrogen tank and a fuel cell stack, where the method includes measuring the pressure in the supply line between redundant shut-off valves at system shut-down and then at system start-up, and where the two pressures are compared to determine if a leak exists.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. The automotive industry expends significant resources in the development of hydrogen fuel cell systems as a source of power for vehicles. Such vehicles would be more efficient and generate fewer emissions than today's vehicles employing internal combustion engines.
A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free hydrogen protons and electrons. The hydrogen protons pass through the electrolyte to the cathode. The hydrogen protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode. The work acts to operate the vehicle.
Many fuel cells are typically combined in a fuel cell stack to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen in the air is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen input gas that flows into the anode side of the stack.
For some vehicle fuel cell system designs, the hydrogen is stored in one or more compressed gas tanks under high pressure on the vehicle to provide the hydrogen necessary for the fuel cell system. The pressure in the compressed tank can be upwards of 700 bar. The compressed tank typically includes an inner plastic liner that provides a gas tight seal for the hydrogen, and an outer composite structural layer that provides the structural integrity of the tank. Because hydrogen is a very light and diffusive gas, the inner liner must be carefully engineered in order to act as a permeation barrier. The hydrogen is removed from the tank through a pipe. At least one pressure regulator is provided that reduces the pressure of the hydrogen within the tank to a pressure suitable for the fuel cell system.
Fuel cell systems of the type discussed above typically have two shut-off valves between the compressed hydrogen tank and the fuel cell stack for safety purposes. A primary shut-off valve is provided in the supply line proximate to the tank or in the tank, and a secondary shut-off valve is positioned downstream from the primary valve. When the fuel cell system is shut down, both the primary and secondary valves are simultaneously closed. Because hydrogen is a small gas, it is sometimes difficult to store the hydrogen without having it leak through valve seals and the like. Further, it is difficult to ensure the seal integrity of the valves over the life of the system. For these and other reasons, it is desirable to have a leak detection system for detecting hydrogen leaks in the fuel cell system, and particularity in the hydrogen supply line.